#include <errno.h>
#include <fcntl.h>
#include <spawn.h>
#include <string.h>
#include <unistd.h>

#include "libc.h"
#include "stdio_impl.h"

FILE* popen(const char* cmd, const char* mode) {
  int p[2], op, e;
  pid_t pid;
  FILE* f;
  posix_spawn_file_actions_t fa;

  if (*mode == 'r') {
    op = 0;
  } else if (*mode == 'w') {
    op = 1;
  } else {
    errno = EINVAL;
    return 0;
  }

  if (pipe2(p, O_CLOEXEC))
    return NULL;
  f = fdopen(p[op], mode);
  if (!f) {
    close(p[0]);
    close(p[1]);
    return NULL;
  }
  FLOCK(f);

  /* If the child's end of the pipe happens to already be on the final
   * fd number to which it will be assigned (either 0 or 1), it must
   * be moved to a different fd. Otherwise, there is no safe way to
   * remove the close-on-exec flag in the child without also creating
   * a file descriptor leak race condition in the parent. */
  if (p[1 - op] == 1 - op) {
    int tmp = fcntl(1 - op, F_DUPFD_CLOEXEC, 0);
    if (tmp < 0) {
      e = errno;
      goto fail;
    }
    close(p[1 - op]);
    p[1 - op] = tmp;
  }

  e = ENOMEM;
  if (!posix_spawn_file_actions_init(&fa)) {
    if (!posix_spawn_file_actions_adddup2(&fa, p[1 - op], 1 - op)) {
      if (!(e = posix_spawn(&pid, "/bin/sh", &fa, 0,
                            (char*[]){(char*)"sh", (char*)"-c", (char*)cmd, 0}, __environ))) {
        posix_spawn_file_actions_destroy(&fa);
        f->pipe_pid = pid;
        if (!strchr(mode, 'e'))
          fcntl(p[op], F_SETFD, 0);
        close(p[1 - op]);
        FUNLOCK(f);
        return f;
      }
    }
    posix_spawn_file_actions_destroy(&fa);
  }
fail:
  fclose(f);
  close(p[1 - op]);

  errno = e;
  return 0;
}